Increasing concern with cost-containment in health care delivery has carried patient adherence issues to the forefront. Adherence to medical recommendations may be the most important mediating mechanism between the therapeutic expertise of the practitioner and outcomes of care. Cost control mechanisms are unlikely to succeed unless rates of noncompliance are substantially lowered. Thus, efforts to enhance adherence hold the promise of reduced costs as well as better outcomes. Despite its therapeutic and practical benefits, long-term adherence to prescribed medical therapy is estimated to be 50% lower. A better understanding of the factors that predict variations in patient compliance is needed to identify features of medical care that should be preserved. We propose to analyze data from the Medical Outcomes Study (MOS), an ongoing investigation that provides a unique opportunity to shed light on adherence issues. The MOS is a major 6-year study of patients with prevalent and treatable chronic conditions (hypertension, diabetes, heart disease and depression) sampled to represent solo and group practices and both prepaid and fee-for-service arrangements in Boston, Chicago, and Los Angeles. The MOS database is characterized by: breadth and depth of explanatory and outcome measures, repeated measurements over a 2-year period, and a large longitudinal sample (N=2, 349) patients sample, 37% of which are 65 or older. The proposed research would extend MOS analyses, without additional data collection, to examine the correspondence between patient reports and physiologic indicators of adherence, determine the extent to which patient adherence varies by provider, evaluate the covariation among adherence and other health-related behaviors, evaluate the associations between adherence and health outcomes, and identify antecedents of patient adherence to medical care. Both general and specific (i.E., to a particular regimen) measures of adherence will be analyzed.